1. Field Of The Invention
The present invention relates to a process for drying conductive material, particularly coal particles and coal fines, using microwave energy. In accordance with the present invention, a fine powdery coal aggregate is dried without using intensely heated gases and risking igniting the fine aggregate.
2. Discussion Of The Prior Art
In the prior art, microwaves have been used to dry and dehydrate non-conducting materials, such as food products. These microwave techniques depend on the presence of polar molecules to induce the heating effect. This is known as dielectric heating. As microwaves pass through the material to be heated, polar molecules, such as water molecules vibrate rapidly as they attempt to align themselves with the constantly changing electric field in the microwave. When a wet conductive material is subjected to microwave radiation, the water molecules are caused to vibrate in the same way they are caused to vibrate with non-conducting materials. However, microwave radiation of conductive materials is accompanied by an additional effect which has led those skilled in the art away from using microwaves to dry conductive material. When a conductive material is subjected to microwave radiation, the atoms which make up the material absorb and conduct the microwave energy so efficiently that arcing can occur, thus igniting the material to be dried, resulting in damage to the product. Unlike non-conducting materials the atoms of which are transparent to microwave radiation, the atoms of conductive materials are more susceptable to the production of free electrons which can be induced by microwave radiation. This enables arcing to occur. Consequently, one skilled in the art would not expect microwaves to be useful in drying conductive materials since the arcing phenomena would be expected to occur, thus rendering the process useless. I have discovered that by reducing the size of conductive particles like coal, I can prevent arcing and thereby dry the material using microwaves.
The application of microwave technology to drying coal has special advantages. After mining, metallurgical coal is pulverized into small particles and passed through a washing plant to remove both ash and sulfur. Subsequently, the aggregate is centrifuged to remove the wash water. After centrifuging, the coal still contains about 15% absorbed water which must be removed before the coal can be used for coking for which the water content must be on the order of 6% or less. Consequently, the coal particles must be dried further and following standard technology this means that intensely heated gases from an electrical or gas furnace are blown through the fine particles in a fluidized bed treatment.
There are several disadvantages to drying coal using intensely heated gases. A source of intensely heated gases must be provided, and this requires large volumes of air and a furnace of some type. Additionally, about 20% of the aggregate ends up at a size less than 28 mesh. The small size of this fraction aggrevates the drying process. As heated gases are blown through the aggregate, the powdery material is picked up by the gases. Unlike the larger particles, the powdery materials create a dust problem. The powdery materials do not settle easily and they tend to coat the inside of the drying apparatus, as well as to be carried out of the apparatus with the exhaust gases. Accordingly, for environmental reasons, special steps must be taken in the drying facility to remove the powder from gases and to contain the powdery material. In addition, the powdery material is extremely flammable. As the heated gases become saturated with the powder, a potentially dangerous condition develops where the slightest degree of overheating could ignite the powder and start an intense fire within the drying facility. Accordingly, the conventional drying must be carefully monitored to prevent the powder from igniting.
In accordance with the present invention, the foregoing disadvantages are overcome. First, the microwave drying facility is principally made up of a microwave generator, waveguides to direct the microwave beam, and a chamber or cavity where the beam irradiates the material to be dried. No furnace or large volume of heated gas is required to operate the microwave facility; however, heated air may be passed through the apparatus to assist in the removal of water vapor from the chamber. Since the material to be dried need not be bathed in the warm air, turbulent mixing is not necessary. Second, microwave energy id directed to the surface of the coal particles where the water is absorbed and consequently where the energy is most effectively used to drive the water from the coal. While the microwave drying process can be assisted by stirring the aggregate, again, it is not necessary to blow the aggregate around and create the dust problem which hampers the conventional technique. Finally, because the microwave energy can be directed to the surface of the coal particles, it is not necessary to generate the intense heat conditions which raise the risk of fire igniting the coal.
The following list of U.S. patents relates to microwave drying of non-conducting materials: U.S. Pat. Nos. 3,409,447, 3,432,636, 4,015,341 (grain and food products); 3,775,860 (timber); 2,483,623, 3,184,575, 3,771,234 (oil and polymer); 3,831,288, 3,997,388 (organic fertilizers); 3,528,179 (generally).